The human eye cannot detect infrared light. But infrared energy can be detected electronically. Sophisticated electronic instruments exist which can scan a scene and convert the infrared light to an electrical signal which can be displayed on a video monitor, analyzed by a computer, or recorded on film. Electrically, the output of these instruments is very similar to the output of a conventional video camera.
Due to their complexity, IR imaging systems are expensive, sensitive, high-maintenance devices. To assure proper operation of these systems and to achieve their full performance requires frequent test and calibration. Engineers, who design IR imaging systems, test them during the design and development stage to evaluate performance parameters and to refine designs to optimize performance. Manufacturers of IR imaging systems need to compare actual performance to specifications, and need to calibrate the systems prior to delivery. End users must test their systems regularly to verify proper operation, and must recalibrate them periodically while they are in the working environment.
Some of the important performance characteristics of an IR imaging system are spatial resolution (ability to resolve fine detail), thermal resolution (ability to resolve small temperature differences), speed (ability to respond to a rapidly changing scene without blurring), and dynamic range (how large a temperature span it can view without saturating). Standard tests have been developed to quantify these characteristics.
IR Test Equipment
Setup, test, and calibration of IR imaging systems requires the use of specialized test equipment. This test equipment is designed to create an infrared scene of precisely known characteristics, to project this scene to the input of the IR imaging system being tested, and to evaluate the quality of the output of the IR imaging system.
Infrared Scene Projector (IRSP)
An Infrared Scene Projector (IRSP) may be used to test a wide variety of sensors used by the US military and major defense contractors. Generally, an IRSP comprises a large number of thermal (IR) emitters, arranged in an array of pixel elements, such as 1024×1024 pixel elements.
The IRSP may use a single chip IR emitter array to produce actual thermal imagery. The emitter array utilizes a large number of pixels to generate the image (similar to how a digital camera uses a large number of pixels to capture an image). Each pixel emits thermal energy that is ultimately captured by the sensor under test. There are many types of emitters such as resistive bridges, Light Emitting Diodes (LEDs), lasers, deformable membranes, micro mirror arrays, etc. Of these emitter types, resistive bridge arrays and micro mirrors are the most widely used. Resistive bridges may offer the best performance in terms of temperature range, speed (frame rate and thermal transition time) and thermal resolution.
Resistive Bridge Pixel Elements
Each pixel element (or “emitter pixel”) of a resistive bridge array generally comprises a heating element (resistor) that is suspended over a temperature controlled substrate. Suspending the resistor keeps the cool substrate from restricting the emitter pixel's temperature range.
Although the resistor and support features for resistive bridge structures may be laid out in different formats, use different materials and have different drive circuitry, the overall structure, and architecture of the pixels are generally always the same. A key aspect of the resistive bridge pixel is that it is, as the name implies, a bridge structure. A resistor, absorber, encapsulant and leg are suspended above the substrate. This suspended bridge performs several functions, among them are thermal isolation from the cool substrate, and formation of a resonant optical cavity. Although resistive bridge structures can be made in many ways, there are several defining characteristics:                1) Thermal resistor suspended as a bridge over an air gap (also known as an optical cavity)        2) Interface to a silicon CMOS chip that provides power to the suspended resistor        3) Use of an absorber layer to increase optical fill factor        4) Use of leg structures to tune thermal conduction to the cooled silicon substrateLarge Arrays        
Fabricating large arrays for imaging applications as well as scene projection is often limited by the yield of the large array. While there is a demand for large arrays, it is typically not economical to produce a large monolithic array, optical or mechanical. Tiling of smaller arrays can improve yield but, in the past, has left seams between the tiles that were comparable to the size of a pixel or larger and introduced unacceptable artifacts in both imaging and scene projection applications.
Bolometer
The techniques disclosed herein for tiling arrays of pixel elements may have applications beyond thermal emitters such as arrays of resistive pixel elements. For example, in creating tiled arrays of thermal detector pixel elements, such as microbolometers.
A bolometer is a device for measuring the power of incident electromagnetic radiation via the heating of a material with a temperature-dependent electrical resistance. A bolometer consists of an absorptive element, such as a thin layer of metal, connected to a thermal reservoir (a body of constant temperature) through a thermal link. The result is that any radiation impinging on the absorptive element raises its temperature above that of the reservoir—the greater the absorbed power, the higher the temperature. A microbolometer is a specific type of bolometer which may be used as a detector in a thermal camera. Infrared radiation strikes the detector material, heating it, and thus changing its electrical resistance. This resistance change is measured and processed into temperatures which can be used to create an image. Unlike other types of infrared detecting equipment, microbolometers do not require cooling.
Through Silicon Vias
A through-silicon via (TSV) is a vertical electrical connection (via)(Vertical Interconnect Access) passing completely through a silicon wafer or die. TSVs are a high performance technique used to create 3D packages and 3D integrated circuits, compared to alternatives such as package-on-package, because the density of the vias is substantially higher, and because the length of the connections is shorter. See, for example, A Study of Through-Silicon-Via Impact on the 3D Stacked IC Layout, Kim et al., ICCAD '09, Nov. 2-5, 2009, incorporated by reference herein as an example of making connections through a semiconductor chip. See also Fabrication and characterization of metal-to-metal interconnect structures for 3-D integration, Huffman et al., © 2009, IOP Publishing Ltd and SISSA, incorporated by reference herein.
Some Prior Art
The following patents and publications are incorporated in their entirety by reference herein.
U.S. Pat. No. 5,600,148 discloses low power infrared scene projector array and method of manufacture. The array combines a two-tier architecture created with special processing whereby each pixel member resides on an elevated platform directly over discrete pixel control electronics and electrically conducting traces couple a plurality of pixels so that they can be controlled to project thermal images at equal to or faster than video frame rates. Microlens assemblies coupled to each discrete pixel improves the thermal efficiency of the array for certain applications. In the method of fabrication, a semiconductor microbridge-type structure obtains with the use of sacrificial layers under deposited pixel members in a compact array so that the pixel electronics reside beneath their associated pixel and the array electronics inhabit the same chip as the array thereby improving fill factor and time constant of the resulting array.
U.S. Pat. No. 7,439,513 discloses fast microbolometer pixels with integrated micro-optical focusing elements. Each microbolometer pixel includes a focusing element located between the pixel body and a substrate, this focusing element preferably sending radiation back towards the central portion of the microbolometer. There is also provided a microbolometer array having a plurality of such microbolometer pixels.
Characterization of the Dynamic Infrared Scene Projector (DIRSP) Engineering Grade Array (EGA), Manzardo et al., SPIE Vol 3697, April 1999, incorporated by reference herein, discloses a 672×544 format suspended membrane microresistor emitter array. Underlying CMOS electronics for the array contain addressing decoder and MUX electronics, as well as integrated sample and hold FET electronics. The DIRSP array is designed for either 32 or 64 parallel analog channel input operation.